1. Field of the invention.
The present invention relates to an apparatus for application of a liquid or pasty coating medium onto a substrate moving past it, for example, a material web of paper, cardboard or boxboard or an applicator roll.
2. Description of the related art.
Known as applicators for application of a liquid or pasty coating medium onto a substrate moving past it, for example, are so-called short-dwell applicators. In such short-dwell applicators the coating medium is passed directly into a coating chamber, or back-up chamber, which on its substrate exit end is bounded by a doctor blade or a roll doctor, while on its substrate entrance end it is closed off by a back-up plate or a further doctor blade. Such short-dwell applicators involve various problems which impede achieving a uniform coating on the substrate moving past.
One problem is that the doctor must be uniformly xe2x80x9csweptxe2x80x9d in order to achieve a uniform coating. That is, the pressure of the approaching coating medium must be so evenly exerted across the entire coating width of the doctor that the doctor lifts off the passing substrate evenly enough to form a clearance of the desired width. In known applicators, however, a sufficiently uniform pressure does not prevail in the coating chamber directly before the doctor. Thus, a relatively great premetering quantity must typically be employed, since otherwise the doctor is unable to guarantee the required minimum coverage of the substrate with coating medium at several spots of the coating width. Another problem is that the uniform coating application on the substrate is hindered by air bubbles which can proceed past the back-up chamber entrance bounding element and into the back-up chamber.
Known from European Patent Document No. EP 0 319 503 B1 is an applicator in which ducts are provided in the back-up chamber entrance bounding element formed by a doctor blade. Through these ducts, the coating medium introduced into the back-up chamber at surplus can exit the back-up chamber in a direction opposite to the direction of travel of the substrate. The exited coating medium collects before the back-up chamber in the area between the substrate and the back-up chamber entrance bounding element, preventing, as a xe2x80x9cseal,xe2x80x9d air bubbles from entering. However, the difficulties discussed in conjunction with the xe2x80x9csweepingxe2x80x9d addressed above also occur with the coating apparatus known from European Patent Document No. EP 0 319 503 B1.
The present invention provides a coating apparatus with which a more uniform coating application can be achieved while at the same time reducing the required premetering quantity. According to the invention, the feed device, viewed in the direction of travel of the substrate, is arranged before the back-up chamber entrance bounding element. With this arrangement of the feed device, the invention finishes xe2x80x9ctwo flies with one swat.xe2x80x9d
Obtained is an accumulation of coating medium in front of the back-up chamber entrance bounding element, the same as in European Patent Document No. EP 0 319 503 B1, which accumulation seals the back-up chamber against the penetration of air bubbles. Moreover, the inventionally achieved sealing effect considerably surmounts the sealing effect accomplished in European Patent Document No. EP 0 319 503 B1, since this accumulation is formed not only by the surplus coating medium, but also by the entire amount of coating medium fed into the back-up chamber. Also, the back-up chamber entrance bounding element represents a choking point which hinders, to a desired and controlled extent, the entrance of coating medium into the back-up chamber. This choking effect occasions in the coating medium accumulating in front of the back-up chamber an equalization in cross direction, i.e., an equalization over the coating width. Consequently, the coating medium entering the back-up chamber possesses a uniform pressure distribution, and a further equalization of the distribution and of the pressure profile of the coating medium in cross direction occurs in the back-up chamber.
Hence, the coating medium prevails in the inventional coating apparatus across the entire coating width on the back-up chamber exit bounding element, e.g., a doctor element, at substantially equal pressure. Thus, the doctor element is inventionally xe2x80x9csweptxe2x80x9d highly uniformly. This makes it possible to achieve a uniform coating application on the substrate. Owing to the pressure-equalizing effect of the arrangement according to the invention, a smaller premetering amount as compared to known apparatuses is sufficient to obtain the desired coating application. A further advantage of the inventional coating apparatus is that the option of working with a slight coating medium surplus makes it possible to keep the purchasing and operating costs of the inventional coating apparatus low. For example, the pump capacity required for circulating the coating medium surplus can be kept accordingly low.
The above effect of the inventional coating apparatus allows further improvement by configuring a back-up chamber wall, which connects the back-up chamber entrance bounding element and/or the substrate entrance end and substrate exit end of the back-up chamber with one another, with a view to equalizing the coating medium in cross direction. To that end, the back-up chamber may feature an equalizing section and a coating section bordering on it. The coating section of the back-up chamber ensures that pressure differences still existing in the back-up chamber, e.g., due to swirling or the like, cannot spread up to the immediate vicinity of the doctor.
In the area of the back-up chamber entrance, equalization in cross direction may be aided, e.g., by configuring the back-up chamber entrance bounding element uneven, viewed in the direction of travel of the substrate. For example, the back-up chamber entrance bounding element can be wavy, stepped or with a predetermined roughness in the area of its greatest proximity to the substrate. A diffuser effect occasioned by the unevennesses is utilized in this case. The diffuser effect can be utilized in the area of the back-up chamber by appropriate design of the back-up chamber wall. Besides the options named above, of a wavy, stepped or rough configuration of the back-up chamber wall, consideration may also be given to the installation or attachment of webs, bars or the like in the back-up chamber.
When operating with a coating medium surplus, flow conditions are possible at which the coating medium enters the back-up chamber near the substrate, in the area of the back-up chamber entrance, in an entrance flow oriented in the direction of travel. The coating medium exits the back-up chamber in a surplus flow, away from the substrate, opposite to the direction of travel. Coating medium which enters the back-up chamber at surplus can exit again through at least one backflow duct. The advantage of such backflow ducts is that they produce defined flow conditions in the area of the back-up chamber entrance formed by the back-up chamber entrance bounding element. The backflow ducts may be, e.g., separate lines or channels traversing the doctor bed. Moreover, the backflow ducts may allow respective opening and closing, selectively and independently of one another.
The back-up chamber entrance bounding element may be, e.g., an element integral with a mounting of the back-up chamber exit bounding element. But it is also possible to have the back-up chamber entrance bounding element form an element which is separate from the mounting of the back-up chamber exit bounding element. The former embodiment variant is distinguished by a simple and robust structure, while the latter variant offers the option of choosing the back-up chamber entrance bounding element in contingence on the properties of the relevant coating medium and on other operating parameters of the coating apparatus, notably the speed of travel of the substrate. Employed as separate back-up chamber exit bounding elements, in particular, are doctor blades, wherein consideration may be given to a configuration such as a drag blade as well as a scraper blade. Scraper blades are forced onto the substrate by the approaching coating medium, since they are oriented opposite to the travel direction of the substrate and rest on it. Such scraper blades may feature at least one entrance duct, possibly near the substrate. In both cases, the above-mentioned backflow ducts may be easily formed by holes made in the blade elements.
Independent of the integral or separate configuration of the back-up chamber entrance bounding element addressed above, the element may feature a channeling surface for the coating medium applied onto the substrate by the feed device and entering the back-up chamber. The channeling, or hopper, surface allows improving the desired pressure equalization in cross direction and safeguarding an orderly entrance of the coating medium into the back-up chamber.
One fluidic influencing of the coating medium which enters or has entered the back-up chamber can be obtained, e.g., by configuring the cavity bounded by the substrate and the channeling surface and/or the back-up chamber wall with a cross section in the fashion of a venturi nozzle.
An edge of the back-up chamber entrance bounding element adjacent to the substrate, viewed in cross direction, may extend, e.g., rectilinearly. The edge may also feature sections jutting out toward the substrate and sections recessed relative to the substrate. An edge of the back-up chamber entrance bounding element adjacent to the substrate may also have at least one entrance duct. Such irregularities and ducts guarantee a desired minimum entrance cross section for the coating medium in the back-up chamber. The latter configuration, moreover, also enables a deliberate de-equalization of the coating medium in cross direction, which results in a corresponding de-equalization of the coating. For example, the edge may be wavy, sawtooth-like, stepped or the like.
The back-up chamber entrance bounding element may be joined fixedly to the mounting of the back-up chamber exit bounding element. It is also possible to arrange the back-up chamber entrance bounding element on the mounting of the back-up chamber exit bounding element in a fashion allowing displacement in relation to it. Further, it is possible to connect the back-up chamber entrance bounding element to the mounting of the back-up chamber exit bounding element by way of a web section which allows a limited tilt movement of the back-up chamber entrance bounding element in relation to the mounting. All of these embodiment variants allow a specific influencing of the back-up chamber, irrespective of the relevant setting of the back-up chamber exit bounding element against the substrate. In the case of the former embodiment variant, the influencing may take place, e.g., by varying the relative orientation of mounting and substrate. In the case of the second and third embodiment variants, a relative movement of the back-up chamber entrance bounding element relative to the mounting may be additionally utilized for influencing the back-up chamber.
Using an actuator device for altering the setting of the back-up chamber exit bounding element against the substrate, e.g., the desired coating clearance size may be preset. Using an actuator device for altering the setting of the back-up chamber entrance bounding element against the substrate, the size of the entrance opening to the back-up chamber and/or the size and shape of the back-up chamber may be influenced. Each of the above actuator devices may include a plurality of actuator units distributed across the substrate width and allowing actuation independently of one another. Thus, a profiling of the back-up chamber entrance opening and/or a profiling of size and shape of the back-up chamber can be accomplished over the width of the coating apparatus, that is, in cross direction.
To achieve a desired coating profile, the pressure cross profile forming before the doctor is significant. To adjust a desired pressure cross profile, the volume of the back-up chamber in cross direction may be profiled. With a constant back-up chamber volume, alternatively, the shape of the back-up chamber cross section in cross direction may be profiled. Also, the cross sections of a plurality of backflow ducts provided side by side in cross direction may be chosen independently of one another to achieve a desired pressure cross profile.
The actuator units may be actuated electrically, hydraulically, pneumatically, hydropneumatically and/or manually. For example, at least part of the actuator units may be formed by adjusting screws. Additionally or alternatively, at least part of the actuator units may be formed by pressure hose units. It is also possible for the actuating system to include a pressure hose subdivided in a plurality of chambers. Remote-controlled actuator units or remote-controlled actuator systems can be incorporated readily in the coating control loop. Furthermore, the adjusting force, or adjusting intensity of the actuator system(s) and/or actuator unit(s) may be controllable or regulatable.
As follows from the preceding discussion, the entrance opening to the back-up chamber, formed by the cooperation of back-up chamber entrance bounding element and substrate, exerts a certain choking effect on the coating medium. The effect results in a premetering of the coating medium. A still finer premetering can be achieved, e.g., with a feed device featuring a plurality of feed elements distributed over the width of the substrate. The rates of coating medium application of these feed elements can be adjustable independently of one another, which is notable in view of achieving a coating profile varying in cross direction.
The invention also relates to a method for the application of a liquid or pasty coating medium onto a traveling substrate, for example, a material web of paper, cardboard or boxboard or an applicator roll. With respect to the advantages achievable with this method, reference is made to the preceding discussion of the inventional coating apparatus.